High-speed rotating cutting tools are used in the process of metal removal. One of the primary goals in the design of end mills is to produce a cutting surface capable of rapidly removing substantial quantities of material while leaving a comparatively smooth surface on the workpiece. It is also desirable to minimize the cutting efforts in order to lower the power requirement for the driving tool.
Smoothness of a surface is obtained if the cutting surface of the end mill remains straight and does not vibrate during use. Vibrations of an elongated straight body create unwanted curvature along its principal axis. Vibrations of a rotating cutting tool, often called “chatter” or “ringing,” are caused by the tool body being excited at its natural frequency or harmonics of this frequency by alternating and rotating load forces. To obtain a smooth surface, it is preferable to design an end mill with limited chatter. An end mill operating at a fixed rotation speed with numerous cutting edges is subject to load forces associated with the removal of layers of material from the workpiece. One solution to this problem is to design a better end mill with a cutting surface for removing material from a surface while removing primary and harmonic load frequencies associated with the tool's own frequency and associated harmonics.
One type of conventional tool is a rotating cutting tool where the radial distance of each cutting edge away from the longitudinal axis of rotation of the tool varies along the cutting height. The cutting edges, if used individually, even if used at high speed, leave an uneven surface on the material. The tools thus require the use of succeeding circumferential cutting edges with similar cutting edges but at varying distances along the longitudinal to semiflat cutting surface.
Another type of conventional end mill as shown in FIGS. 1A to 1C has cutting edges that do not vary away from the longitudinal axis but are arranged in a helix shape disposed at regular steps A along the circumference of the end mill. These types of cutting edges are regular and contact the material at a fixed angle determined by the cutting angle of the helix. The resulting load forces on each cutting edge are the same at different time intervals and result in the creation of resonating effects at certain cutting speeds.
Still another type of conventional end mill as shown in FIGS. 2A to 2C has cutting edges that are not disposed at regular distances along the circumference of the end mill. As a result, while the direction of the load forces remain perpendicular to the cutting edge, and thus perpendicular to the cutting angle, the time intervals between changes alternate a first load frequency.
Yet still another type of conventional end mill as shown in FIGS. 3A to 3C shows that some of the cutting angles of the cutting edges are modified. This configuration allows for the attenuation of a second type of load frequencies associated with the direction of the load forces on the cutting edge. This change results in the transfer of a varying quantity of energy in the longitudinal axis. Nevertheless, this prior art maintains a constant cutting angle for each cutting edge. Understandably, as the end mill rotates, the load force moves up the cutting edge as the cutting surface removes material and the load force, as it evolves up the longitudinal axis, remains constant and thus creates unwanted resonance.
Another type of conventional end mill as shown in FIGS. 4A to 4C tries to limit a third type of load frequency by creating a plurality of small waves cut into the cutting edge in an attempt to modulate the load force as it migrates up along the cutting surface in the longitudinal axis. This configuration does not remove all unwanted frequencies since the load force alternatively evolves along cycles. In fact, it creates a new type of cycling load associated with the pitch of the small waves. In FIG. 4C, along the cutting edge 4a, the load force will return to the same angle four times as it progresses along the length of the fluted area. The effective cutting angle rapidly evolves over a short distance, creating unwanted shear forces in the longitudinal axis that increase the cutting effort needed on the tool, which may also damage the cutting edge.
What is needed is an end mill that reduces unwanted vibrations based on creation of noncyclic loading. A nonchattering or harmonically stable tool would limit damage to the end mill, provide a more polished finished surface, and limit the cutting effort needed to operate the cutting tool. The end mill must also be easy to manufacture and not present local weaknesses to shear forces.